Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Saturday, March 3, 2012

Puzzle of European hair and eye color

Hair and eye color diversity is unusual in two ways. It’s confined to Europeans. And it seems to be linked to prenatal feminization.Europeans are distinguished from other humans by a diverse palette of eye and hair colors. I’ve argued that these color traits arose from intense sexual selection of women in ancestral European environments (Frost, 1994, 2006, 2008). Until 10,000 years ago, Europe had vast expanses of continental tundra—an environment where male hunters provided almost all the food and where long-distance hunting caused more deaths among young men than among young women. With fewer men overall, and fewer who could shoulder the costs of polygyny, the mate market had a surplus of unmated women. The pressure of sexual selection was therefore on the female sex. They were the ones who had to compete for mates.

East of Europe, the tundra zone swung north into colder and more arid territory. Humans were present there too, but in fewer numbers and less continuously. Only in Europe did this zone have a continuous human presence throughout the last ice age, and only in Europe could humans pass on and steadily accumulate the genetic legacy of intense female-directed sexual selection.

But if sexual selection had created European hair and eye colors, wouldn’t we see some sex linkage? Wouldn’t these color traits be expressed more in women than in men?

When I initially wrote on this subject, I had some evidence of sex linkage. Blond hair darkens with age more slowly in women than in men (Olivier, 1960, p. 74). I had also read an unpublished study that found higher digit ratios, and thus higher prenatal exposure to estrogen, in individuals with blond hair or non-brown eyes (Mather et al., unpublished).

But that was it. So I fell back on the explanation that sex linkage would have taken too long to evolve, especially because men incurred little cost in having unnecessarily showy eye and hair colors.

I’ve since come across more evidence of sex linkage. First, a twin study has shown that hair is, on average, lighter-colored in women than in men, with red hair being especially more frequent in females. Women also show greater variation in hair color (Shekar et al., 2008).

Second, blue-eyed men seem to have a more feminine face shape. This was the unintended finding of a rating study of male facial photos, which initially found that brown-eyed men were perceived to be more dominant than blue-eyed men. As a control, the authors repeated the experiment after altering the photos to give the brown-eyed men blue eyes. The altered photos were still rated as more dominant. Careful study revealed that the brown-eyed men had more masculine faces with broader and more massive chins, broader mouths, larger noses, larger eyebrows, and closer-set eyes. The blue-eyed men had smaller and sharper chins, narrower mouths, smaller noses, and a greater span between the eyes. It was thus face shape and not eye color that made the brown-eyed men seem more dominant. The authors denied the possibility of ethnic differences between the two groups, stating that the photos depicted only university students of Czech origin (Kleisner et al., 2010).

Finally, a study of preschool children suggests that blue eyes are sex-linked to shyness:

In the present study, 152 Caucasian preschool-aged (Mage=54.09 months, SD=5.84) children (77 males) with either blue (n=84) or brown (n=68) eyes, were compared in terms of parental and teacher ratings of social wariness, social play, and aggression. A significant Eye Color×Gender Interaction was found in terms of indices of social wariness; blue-eyed males were rated as more socially wary than brown-eyed males, while blue- and brown-eyed females did not differ in this regard. (Coplan et al., 1997)

This finding seems robust in the sense that other authors (cited by Coplan et al.) have found similar results. Although shyness in itself is not more common in girls than in boys, girls show a stronger interaction between loneliness and aggression (Coplan et al., 2007). In a male brain, this interaction might lead to a higher likelihood of social withdrawal, given the generally higher level of aggression in boys than in girls.

But how does this sex linkage operate? It may be mediated by prenatal exposure to estrogen, as Mather et al found in their unpublished study. This pre-natal estrogenization would be over-determined in women, i.e., almost all women are fully exposed to estrogen before birth regardless of their eye color. In men, however, it would be limited to blue-eyed individuals. Thus, blue-eyed men may be partially feminized not only in their face shape but also in their behavior.

Could other behaviors be partially feminized in blue-eyed or fair-haired men? Sexual orientation comes to mind, but that’s one thing that natural selection should have quickly rectified (as Greg Cochran has often pointed out). Indeed, Ellis et al. (2008) failed to find any significant relationship between sexual orientation and eye color or hair color.

Alternate explanations

- Neanderthal admixture?

Not many alternate explanations are still in the running. For a while, a plausible one was Neanderthal admixture. We’ve since managed to retrieve the Neanderthal MC1R allele and it doesn’t match any existing allele in modern humans. In any case, the estimated Neanderthal admixture of 1 to 4% in modern Europeans is well below the frequency of non-black hair and non-brown eyes.

- Genetic linkage to lighter skin?

Perhaps eye and hair color diversity is genetically linked to lighter skin. Eyes and hair have diversified in color in the same part of the world where skin color has lightened the most. Perhaps this lightening involved genes that also affect hair and eye color.

Yet the genes are different in each case. European skin has lightened mainly through replacement of alleles at two genes: SLC45A2 (AIM1) and SLC24A5. European hair color has diversified through a proliferation of new alleles at the MC1R gene. European eye color has diversified through a proliferation of new alleles in the HERC2-OCA2 region and elsewhere.

Lighter skin is associated with a few of the new alleles, namely the ones for red hair or blue eyes. Conceivably, these two color variants are a side effect of selection for lighter skin. But why would such selection increase the total number of alleles for hair and eye color? This is especially strange because many of the alleles have little or no effect on skin pigmentation. And why have neither red hair nor blue eyes reached fixation in any population, even those with milk-white complexions?

- Cochran’s theory

Greg Cochran has argued that European hair and eye color diversity reflects an underlying behavioral polymorphism, which in turn is due to a process of self-domestication that resulted from the advent of agriculture. As Europeans formed larger and more sedentary communities, they had to become more obedient to authority. An extra dose of prenatal estrogen might have been the necessary quick fix to tame European males.

Greg’s theory is a mirror image of my own. I argue that sexual selection of women was driving the evolutionary change. The effects on male behavior are thus merely collateral damage. Greg argues that the evolutionary change was driven by natural selection for male submissiveness. The new hair and eye colors are thus merely a cute side effect.

To my knowledge, Greg has presented this theory at length only in the book he published with Henry Harpending The 10,000 Year Explosion. Even there, the presentation is a bit hazy. The two authors start off well enough:

[…] selection on genes affecting skin color, eye color, and hair color somehow created lots of variety in Europeans: redheads and blondes, blue eyes and green eyes. Nowhere else in the world is that sort of variety common. In most parts of the world, even in temperate regions, everyone has dark eyes and dark hair. (Cochran & Harpending, 2009, p. 94)

This pattern points to “something fundamentally different in the selective forces.” The next few pages suggest that this “something” is selection for submission to authority, which in turn is incidentally linked to eye and hair color. But Cochran and Harpending never spell out the link. The closest they come is a paragraph eighteen pages onward:

Selection for submission to authority sounds unnervingly like domestication. In fact, there are parallels between the domestication in animals and the changes that have occurred in humans during the Holocene period. In both humans and domesticated animals, we see a reduction in brain size, broader skulls, changes in hair color or coat color, and smaller teeth. (Cochran & Harpending, 2009, p. 112)

That’s it. There are no references to the literature linking eye color to behavior. Why? Maybe it’s bad karma to imply that blue-eyed men are more civilized. That sounds a bit …. well, you know.

Personally, I don’t care whether or not a theory is socially acceptable. Only one question concerns me. Does it make sense?

Let’s walk through it step by step. Agriculture created a selection pressure for more obedient men. This selection in turn led to blue eyes and fair hair as a side effect. We know that agriculture spread through Europe in stages. For a long time, the wave of advance stalled along a line stretching from the Low Countries to the Black Sea. Then, around 7,500 years ago, agriculture resumed its northward spread. The last refuge of European hunter/fisher/gatherers seems to have been in the East Baltic, where farming became dominant only 3,000 years ago.

These last holdouts were presumably less affected by the process of self-domestication via agriculture. They should therefore have lower incidences of blue eyes and fair hair. Is that what we see?

Nope.

Greg’s theory has other weaknesses. Are farmers really more submissive than hunter-gatherers? Not necessarily. It isn’t so much farming that makes men less unruly as State formation, specifically the State’s monopoly on the use of violence. That’s a later historical development (Frost, 2010).

And blue eyes are linked not so much to male submissiveness as to male shyness. The two aren’t really the same thing. Can one confidently argue that male shyness is more adaptive among farmers? Aren’t social relations more extensive and intensive in farming communities than in small bands of hunter-gatherers?

23 comments:

Anonymous
said...

"Hair and eye color diversity is unusual in two ways. It’s confined to Europeans."

Come on. You know that it's not confined to Europeans. It can also be found in West and Central Asians, even if darker hair and eyes are more prevalent among said populations. Central Asians are of course Euro/West Asian/East Asian hybrids, and West Asians cluster more closely with Europeans -- the genetic differentiation between them is clinal, and not nearly as sharp and distinct as that between Euros and other populations.

Come on. You know that it's not confined to Europeans. It can also be found in West and Central Asians

The Kalash in Afghanistan comes to mind. Your right Anon, this hair and eye color diversity is not at all confined just to Europeans, but rather it just rises to a higher frequency in the latter population..

But if sexual selection had created European hair and eye colors, wouldn’t we see some sex linkage? Wouldn’t these color traits be expressed more in women than in men?

Let's not omit contradictory evidence though. Blue eyes are more common in men than in women. The temperamental correlations are also found in men. This could be indicative of a selection pressure on men.

Actual selection for light coats and domestication in animals, seems to work the opposite way. "It wasn't that the animals behaved differently," as Andersson says, "it's just that they were cute". Here.

The " reduction in brain size, broader skulls," came about in the Magdalenian, long before the Neolithic surely.

Shyness is social withdrawal, that can not be anything but a serious disadvantage for a man.

The " reduction in brain size, broader skulls," came about in the Magdalenian, long before the Neolithic surely.

What would make you think there was any connection to the Magdalenian? Reduction in cranial capacity is a trend of only the last 10000 years and happened more or less everywhere and was accompanied by brachycephalisation in Eurasia.

"Additionally, although these early samples are small, Mesolithic Europeans had larger endocranial volumes than Upper Paleolithic Europeans, across the same interval when they underwent a substantial decline in stature. That Mesolithic change in endocranial volume is in the opposite direction expected from the change in stature."

Shyness is social withdrawal, that can not be anything but a serious disadvantage for a man.

Autistics may have advantages, under some circumstances at the very least, even besides avoiding conflict.

Has Peters Hypothesis of sexual selection of European women on the paleolithic tundra been nullified by recent gene analysis. I have not saved the links but the date of origin for European hair and eye color alleles based on the number or nearby mutations, were all less than 10,000 years old.

My pet completely unsubstantiated or supported hypothesis is that: The cult which build the stone circles like stone henge etc was part of broader solar religion. Dominated by a priesthood descended from the gods as evidenced by their eyes as blue as the sky, hair as golden as the rays of sunlight, or as red as fire or bronze. AS such they had access to numerous concubines spreading their genes far and wide.

1) side question: what exact color is flaxen hair ?(a merovingian prince was described by romans as a teen with stricking flaxen hair)

2) I don't understand sexual selection if the selected facial features are only an indication of cuteness or non agressivity and wihtout any selective benefits.A hunter or a warrior doesn' t chooses a woman for her genes, but for the genes he want to see in his male offsping. He wants smarter and/or more robust kids, in short, good hunters or good warriors. That's what he wants. Of course, if the mother is also cute, that's just better. But sexual selection for cutness/neoteny alone cannot go against this natural selection andthe selected neotenic traits must have been an indication for something else than just being cute. Being smart, courageous, collaborative, skilled with weapons, etc., certainly outweights brute physical robustness in hunting or at war, so pick up your choice.

3) Anonymous, your priesthood theory is very interesting, but it would apply only to a minority. For the selected characters to spread outside a few weirdo or priviliged individuals fed by the community, point 2 above must apply.

Anon, 10,000 Year Explosion says the peak of brain size in early Europeans was 20,000 years ago, so domestication selected by civilization can't be responsible for brain size and other changes, like smaller jaws (Fist impacted tooth was discovered in 'Magdalenian Girl')

You can't get away from the fact that European women are highly rated for attractiveness. If this is is just a side effect fo domestication domesticated animals should look better than wild ones.

Changes brought about by domestication in animals do not make for then look good. The exception is racehorses, but those have been bred for a 'wild' characteristic - speed.

Domesticated animals don't look good unless they have been bred for something other than domestication. Mere selection for domestication would result in a human version of a Pot-bellied pig.

Anon, 10,000 Year Explosion says the peak of brain size in early Europeans was 20,000 years ago, so domestication selected by civilization can't be responsible for brain size and other changes, like smaller jaws (Fist impacted tooth was discovered in 'Magdalenian Girl')

Sean, I've posted a link to an actual paper by Hawks that indicates peak brain size in the Mesolithic. If possible I'd like to see the quote with page reference in 10,000 Year Explosion that indicates that reduction predates the Holocene, since that isn't what the section quoted from that book by Peter indicates "in humans during the Holocene period. In both humans and domesticated animals, we see a reduction in brain size, broader skulls, changes in hair color or coat color, and smaller teeth."

Yes, it's clinal, but the frequency of non-black hair and non-brown eyes drops very sharply as one moves south and east.

Jason,

My point was that women display more color diversity than men do. This may be as true for eye color as it is for hair color (although no one has done an adult twin study on eye color yet). Women are less likely to have blue eyes but are more likely to have green eyes.

I don't follow your point about temperament correlations. Shyness is neither a female trait nor a male trait. There is however a stronger correlation between shyness and aggressiveness in girls than in boys. I suggested that a partially feminized male brain may be more prone to shyness because men are more predisposed to aggressiveness.

Juoni,

No, I don't. There are other people (like Greg Cochran) who argue that hair and eye color are proxies for something else. But that isn't my argument.

Anon,

The alleles are different. Blondism in Australian Aborigenes seems to be sex-linked, i.e., more expressed in women, and is centered on the western desert. This may have been another case of sexual selection (due to high mortality among hunting males).

Stephen,

You're mistaken if you're referring to the Eiberg et al. 2008 paper. They simply suggested that the new eye colors may date to "about 6–10,000 years ago." There was no real analysis underlying that figure. Greg and Henry made the same mistake.

Ben,

Flaxen = pale yellow

If you're in a buyer's market, you have an "embarras du choix." It's like a movie producer who has too many good candidates. At that point, even trivial things can make a big difference, especially things that can catch and retain attention.

Sean, Anon,

I suspect that brain size did decrease when Europeans went from hunting to farming. Hunting requires an ability to store huge amounts of spatio-temporaral information. In contrast, farming takes place over a much smaller territory where movement is much more predictable.

Hawks mentions "Less well known is that the terminal Pleistocene and Holocene (ca. 30,000 years ago to present) witnessed a substantial decline in endocranial volume" He goes on to mention European females as the fastest shrinking brains. Why would that be?

Here for modern European brains being bigger (15 -10%) just before the steppe-tundra period.

Cro Magnons were considerably larger than their hunter gatherer descendants in Europe. They had robust skeletons and broad faces. They did not have white skin.

Compared to men, women are smaller with less massive and robust skeletons. Proportionately, women have smaller faces (especially jaws) than men. Women are also paler than men.

Article here says max modern human brain size was 20,000 years ago. (In Europe though it doesn't mention this). It quotes a scienist alluding to the domestication/reduced aggression theory. But remember, Hawks noted the fall in brain mass was best exemplified by the fall in women. Selection against aggression would mainly impact on women? No it would not, but sexual selection of women would.

In the case of eye and hair color, sexual selection tends to increase diversity. It selects for those colors that are less common and selects against those that are more common.

To resolve this question, we really need a twin study of adult eye color. If I look at the chart that Jason linked to, it looks as though frequent eye colors are less common in women than in men, whereas rare eye colors are more common in women.

What I meant with the feminization comment was: in a population with blue eyes being the dominant eye colour, and if we take for granted that sexual selection is driving eye-colour diversity in women, then (if anything), shouldn't blue eyes be associated with more masculine traits, whereas brown, black, green, and so forth be associated with feminization?

If sexual selection of women drove eye-colour diversity in Europeans of both sexes, and the colors became sex- linked. I think it follows that men with 'black' and brown eyes would be more masculine than men with blue eyes, but men with green eyes would be less masculine than blue eyed men.

Maybe red hair, in men, would also be associated with less masculinity. No Hollywood leading man was red haired, except for Charles Bickford, in the days of B&W.

Peter, Why assume that reduced masculinity, IE anything less than the most extreme reproductive drive, would be selected against. In practice it might be a trade off with abilities conferred by being less masculine bringing increased resource provisioning ability. A man who was less masculine can be a more capable provider, and be regarded as a 'catch'. The provision of resources to ensure the survival of the children is the key factor in fitness for much of historical time.

IMO G.Cochran's argument only works with the assumption there is no resource garnering difference between full on aggressive heterosexuality and whatever it is being compared to.

G.Cochan's argument is really about partial to complete non-heterosexuality as compared to heterosexuality. Hetrosexual men having with reduced interest in the opposite sex, as a side effect (of sexual selection or whatever) is not explicitly considered. Cochran deals with the resource question by saying that homosexuals are no more intelligent. However, there are other resource garnering differences possible between full on aggressive heterosexuality and a lower powered version of same. Reduced masculinity could make men less 'difficult' and more reliable for example. Less likely to try and cuckold friends and more loyal and trustworthy as an underling.

Sexual selection might have played a big part in priming Europeans for civilization.

According to the study by Mather et al., digit ratios were higher for all non-brown eye colors. There was no evidence that pre-natal estrogenization was stronger, for instance, with green eyes than with blue eyes. On the other hand, the sample was probably too small to make that kind of fine distinction.

Sean,

I'm not sure Greg would accept your description of his argument (he has his own theories about the origins of male homosexuality). I believe Ed Miller was the one who argued that increased paternal investment was brought about by partial estrogenization of the male brain.

It may be that this partial estrogenization has created a susceptibility to male homosexuality and that this susceptibility has been exploited by the hypothetical 'gay pathogen' (à la Cochran) or by environmental estrogens (as I've argued).

If natural selection inexorably eliminates non-heterosexuality then why are there so many heterosexual non reproducing men? Male shyness.

Hetrosexual men having reduced impulse to engage in male-male competition explains it I think. 'Brian Hare, an anthropologist at the Duke University Institute for Brain Sciences' "A smaller brain is the signature of selection against aggression,[...] "Another way to say that is an increase in tolerance. [...] Cro-Magnon man, who lived in Europe 20,000 to 30,000 years ago, had the biggest brains of any human species." According to the article humans have lost a chunk of brain the size of a tennis ball.

While Cro Magnon hunters might have required an ability to store huge amounts of spatio-temporal information Steppe Tundra hunters surely must have been selected for storing even more spatio-temporal information, anyway the changes came long before civilization.

With an excess of females there feminization of physique and brain size would be no great disadvantage as there was a reduced need for aggression to drive off rivals.

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.